1. Field of the Invention
The present invention relates to a gain control amplifier.
2. Description of the Background Art
FIG. 1 shows an example of a structure of a conventional gain control amplifier 100. A signal input terminal 1 is grounded through resistors 2, 3 and 4 which are connected in series and which have resistances R1, R2 and R3, respectively. A contact 6 is connected to a junction of the signal input terminal 1 and the resistor 2. A contact 7 is connected to a junction of the resistor 2 and the resistor 3 while a contact 8 is connected to a junction of the resistor 3 and the resistor 4.
The contacts 6 to 8 and an input terminal of a gain control amplifier 9 form a switch 5 which selects a contact for connection to the input terminal of the gain control amplifier 9. An output terminal of the gain control amplifier 9 is connected to a signal output terminal 11. A control input terminal of the gain control amplifier 9 is connected to a variable voltage source 10.
A signal received at the signal input terminal 1 is voltage-divided by the resistors 2, 3 and 4 so that voltage-divided signals are available at the contacts 6 to 8. One of the voltage-divided signals is admitted to the input terminal of the gain control amplifier 9 by the switch 5 and amplified with a predetermined gain. The gain of the gain control amplifier 9 is responsive to a potential given to the control input terminal of the gain control amplifier 9; that is, the gain of the gain control amplifier 9 is controlled by increasing or decreasing a voltage supplied by the variable voltage source 10. Hence, the gain of the gain control amplifier 100 is roughly controlled by the switch 5 which selects one of the signals which are voltage-divided by the resistors 2 to 4 and finely controlled by the variable voltage source 10.
The gain control amplifier 100 inherently has a weakness that its gain does not always make a continuous change in response to a voltage generated in the variable voltage source 10. FIG. 2 is a graph of a voltage at the variable voltage source 10 versus the gain of the gain control amplifier 100. The characteristic curves 12, 13 and 14 represent voltage/amplification-degree relations of where the switch 5 selects the contacts 6, 7 and 8 for connection to the input terminal of the gain control amplifier 9, respectively.
As is evidenced in FIG. 2, the gain of the gain control amplifier 100, which is dependent on a voltage at the variable voltage source 10, changes discontinuously at around values G1 and G2 due to switching action of the switch 5. Further, it is impossible to make the characteristics represented by the characteristic curves 12 to 14 different from each other since the gain control amplifier 9 alone amplifies the signals independently of the switching behavior of the switch 5.